1. Field of the Invention
The present invention relates to a control device of an internal combustion engine that includes a first fuel injection mechanism (in-cylinder injector) for injecting fuel into a cylinder and a second fuel injection mechanism (intake manifold injector) for injecting fuel into an intake manifold or an intake port, and more particularly to a technique to determine when to inject fuel from the second fuel injection mechanism in homogenous combustion.
2. Description of the Background Art
An internal combustion engine provided with an intake manifold injector for injecting fuel into an intake manifold of an internal combustion engine and an in-cylinder injector for constantly injecting fuel into a combustion chamber thereof, in which fuel injection from the intake manifold injector is stopped when load of the internal combustion engine is lower than preset load and fuel injection from the intake manifold injector is allowed when load of the internal combustion engine is higher than the preset load, has been known.
In addition, among the internal combustion engines, an internal combustion engine switching between stratified charge combustion and homogenous combustion in accordance with an operation state thereof has been known. Here, stratified charge combustion refers to such combustion that fuel is injected from the in-cylinder injector in the compression stroke to form a concentrated and stratified air-fuel mixture around a spark plug, thus achieving lean combustion of the fuel. Meanwhile, homogenous combustion refers to combustion of fuel in such a manner that the fuel is diffused in the combustion chamber to form a homogenous air-fuel mixture.
Japanese Patent Laying-Open No. 2001-20837 discloses a fuel injection control device of an engine, the engine switching between stratified charge combustion and homogeneous combustion in accordance with an operation state and having a main fuel injection valve injecting fuel directly into a combustion chamber and an auxiliary fuel injection valve injecting fuel into an intake port of each cylinder. The fuel injection control device of the engine switching between stratified charge combustion and homogeneous combustion in accordance with the operation state includes the main fuel injection valve injecting fuel directly into the combustion chamber and the auxiliary fuel injection valve injecting fuel into the intake port of each cylinder, and a ratio of fuel injection amount between the main fuel injection valve and the auxiliary fuel injection valve is set as variable based on the engine operation state.
According to the fuel injection control device of the engine, stratified charge combustion is carried out solely with the fuel directly injected from the main fuel injection valve into the combustion chamber, and homogenous combustion is carried out using both of the main fuel injection valve and the auxiliary fuel injection valve (in some cases, solely the auxiliary fuel injection valve is used), so that a capacity of the main fuel injection valve can be small even in a high-output engine. Therefore, linearity of injection period-injection amount characteristic of the main fuel injection valve in a low load region such as idling is enhanced, satisfactory stratified charge combustion can be maintained as a result of improvement in accuracy in injection amount control, and stability of a low-load operation such as idling is improved. Both of the main fuel injection valve and the auxiliary fuel injection valve are used in homogenous combustion to make use of advantages of direct fuel injection and advantages of intake port injection, thus also maintaining satisfactory homogenous combustion.
In the fuel injection control device of the engine disclosed in Japanese Patent Laying-Open No. 2001-20837, stratified charge combustion and homogenous combustion are appropriately selected depending on a situation. Accordingly, control of ignition, injection and throttle becomes complicated, and a control program adapted to each combustion manner is required. In particular, in switching between combustion manners, such control should significantly be modified and it is difficult to carry out excellent control (fuel efficiency, exhaust purifying performance) at the time of transition. In addition, as a three-way catalyst does not function in a stratified charge combustion region where lean combustion is conducted, a lean NOx catalyst should be used, which leads to increase in cost.
In view of these facts, an engine which requires neither control in switching between stratified charge combustion and homogenous combustion nor the expensive lean NOx catalyst but carries out, during a normal operation, homogenous combustion in all regions (except for an abnormal state, that is, rapid catalyst warm-up) instead of stratified charge combustion has been developed.
In such an engine, however, homogeneity is low in a region where engine speed is low and load is high, and torque fluctuation may become significant. In addition, if the fuel is injected from the intake manifold injector in the intake stroke and introduced in the combustion chamber in a liquid state, deposits are accumulated in an injection hole of the in-cylinder injector. The disclosure of Japanese Patent Laying-Open No. 2001-20837 merely states that a ratio of injection from the auxiliary fuel injection valve injecting the fuel into the intake port is set such that it is increased with the increase in engine output (engine speed, load) in the region where homogenous combustion is conducted, and Japanese Patent Laying-Open No. 2001-20837 does not solve the above-described problems.